1. Technical Field
The present invention relates to a thermal conductivity detector, more specifically relates to a thermal conductivity detector capable of realizing high detection performance even with the use of a miniaturized heating element, and expanding an effective applicable temperature range of a heating element, and to relates to a gas chromatograph using the same.
A thermal conductivity detector (TCD) has been in use as the most versatile detector of a gas chromatograph. In the case of the gas chromatograph, a carrier gas such as He, H2, N2, Ar, and so forth is caused to flow thereto, and a measurement gas, as weighed, is introduced thereto to pass through a column, thereby splitting the measurement gas into its components over time to be measured by the detector. Qualitative analysis is conducted on the basis of an occurrence time of an output peak, and quantitative analysis is conducted on the basis of a peak area. The thermal conductivity detector converts a difference in thermal conductivity between a gas component, split off in the column, and a reference gas identical in species to the carrier gas, into an electric signal, thereby detecting respective gas components as split off, and concentration thereof.
2. Related Art
FIG. 6 is a block diagram showing a principle underlying operation of a thermal conductivity detector. In FIG. 6, reference numerals 1 to 4 denote first to fourth cells, respectively; first to forth heating elements, 1a to 4a, are each housed in the first cell 1 to the fourth cell 4, respectively. A measurement gas is introduced from an introduction port 5a of the first cell 1 to flow through the respective interiors of the first cell 1, and the second cell 2 to be outputted from an output port 5b of the second cell 2 while a reference gas is introduced from an introduction port 6a of the third cell 3 to flow through the respective interiors of the third cell 3, and the fourth cell 4 to be outputted from an output port 6b of the fourth cell 6. A bridge circuit 7 is made up of the first to fourth heating elements, 1a to 4a. A predetermined current from a constant current source 8 is fed to the bridge circuit 7, thereby causing the first to fourth heating elements, 1a to 4a, to generate heat, respectively. The measurement gas takes heat away from the heating elements 1a, and 2a, respectively, and the reference gas takes heat away from the heating elements 3a, and 4a, respectively. As a result, temperature of the respective heating elements will vary due to a difference in thermal conductivity thereof, thereby causing a change in resistance value thereof, so that an unbalanced voltage occur to the bridge circuit 7. Upon a detection circuit 9 detecting the unbalanced voltage, an amount of variation in thermal conductivity of the measurement gas is measured.
FIG. 7 is a sectional block diagram showing the principal part of a conventional sensor for use in the thermal conductivity detector described as above. In FIG. 7, reference numeral 10 denotes a block made of aluminum or stainless steel. First and second through-holes, 11, 12, in parallel with each other, are formed in the block 10, and heating elements 13, 14, made up of a filament, respectively, are disposed in the first through-hole 11, and the second through-hole 12, respectively.
Further, first to fourth inner flow-paths, 15a to 15d, extended from respective flow-inlets 11a, 12a of the first through-hole 11, and the second through-hole 12, in respective directions at an angle 45° from the first through-hole 11, and the second through-hole 12, respectively, are formed, while fifth to eighth inner flow-paths, 15e to 15h, extended from respective flow-outlets lib, 12b of the first through-hole 11, and the second through-hole 12, in respective directions at an angle 45° from the first through-hole 11, and the second through-hole 12, respectively, are formed.
The inner flow-paths, 15e to 15h, are bonded with each other to form a flow-path in a shape resembling the letter W, and the flow-path W is further bonded with the first inner flow-path 15a, and the fourth inner flow-path 15d, respectively, thereby forming respective bypass flow-paths of the first through-hole 11, and the second through-hole 12, when the first through-hole 11, and the second through-hole 12 each serve as a main flow-path. Reference numeral 18a denotes a fluid inflow pipe, and 18b a fluid outflow pipe, these pipes being reinforced by reinforcing members 19a, 19b, respectively. Reference numerals 20a, 20b, 22a, 22b each denote a lead wire, and the lead wires 20a, 20b, 22a, 22b are sealed by hermetic seals 21a, 21b, 23a, 23b, respectively.
With the detector of such a configuration as described, when a predetermined fluid, such as a measurement gas, or a reference gas is fed to an introduction hole 16, a flow of the predetermined fluid passes through the introduction hole 16 to be subsequently split into two flows, and each flow proceeds through the second flow-path 15b, and the third inner flow-path 15c, respectively. Further, a flow having passed through the second inner flow-path 15b is further split into two flows, each flow moving through the first through-hole 11, and the first to fifth inner flow-paths, 15a, 15e, respectively, and subsequently, respective flows are merged with each other to form a flow, whereupon the flow moves through the sixth flow-path 15f. 
A flow of the predetermined fluid, moving through the third inner flow-path 15c, is similarly, split into two flows, each flow moving through the second through-hole 12, and the fourth to eighth inner flow-paths 15d, 15h, respectively, to be subsequently merged with each other before moving through the seventh inner flow-path 15g. Further, the respective flows of the fluid, moving through the sixth inner flow-path 15f, and the seventh inner flow-path 15g, are merged with each other to be delivered outside the block 10 via a delivery hole 17.
The blocks configured as above are provided to be used for the measurement gas, and the reference gas, respectively, and an unbalanced voltage according to a difference in thermal conductivity between the measurement gas, and the reference gas is drawn out. The heating elements 13, 14, shown in FIG. 7, correspond to the heating elements 1a, 2a, in FIG. 7, respectively (to the heating elements 3a, 4a, in FIG. 6, in the case of the block through which the reference gas is circulated).
For the heating element, a filament coil with a total wire length in a range of several to several tens of cm has so far been in heavy use, however, there has lately been adopted a practice to form a miniaturized sensor of the thermal conductivity detector on a substrate with the use of the techniques of MEMS (Micro Electro Mechanical System). As a specific example, a practice has been adopted whereby flow-paths are formed in the interior between two substrates stuck with each other, and a filament is miniaturized to be formed on one of the substrates such that the filament is disposed in the space of each of the flow-paths.
FIG. 8 is a view showing an example in which a gas flow-path, and a filament are formed on a substrate by use of the MEMS techniques. In FIG. 8, reference numeral 10 denotes a silicon (Si) substrate, the silicon (Si) substrate serving as one half part of an inner wall enclosing the filament. Reference numeral 20 denotes a Pyrex (registered trademark) (Px) glass substrate, and a beam 11 extended in the direction of a line Y is formed on the Pyrex glass substrate. A Pyrex glass represents one example of borosilicate glass. The beam 11 is formed in a slender shape having a width w, and a thickness d. Regions 12, 13, spreading so as to be rectangular in shape, respectively, are disposed on the sides of respective ends of the beam 11, these regions, and the beam 11 being made of material prepared by adding an additive to silicon, and adjusted so as to cause a resistance value to be reduced. The beam 11 functions as the filament, and the regions 12, 13 each function as the electrode connected to the filament. The Pyrex glass substrate 20 positioned underneath the regions 12, 13 is provided with a through-hole (not shown), and the electrodes are taken out through this through-hole.
The Pyrex glass substrate 20 is fixedly attached to the silicon substrate 10 in such a way as to be superposed therewith from the underside in the figure by, for example, anodic bonding. A hollow part 21 is formed in the central region of the Pyrex glass substrate 20, and when the Pyrex glass substrate 20 is fixedly attached to the silicon substrate 10, the beam 11 to serve as the filament will be disposed just in the space of the hollow part 21. The hollow part 21 is adjacent to a flow-path (not shown), and heat of the filament is taken away by a gas diffusing inside the hollow part 21.
In the case where the sensor of the thermal conductivity detector is formed by use of the MEMS techniques, as described, there can be derived merits including;                because the flow-path, and the filament can be formed by a semiconductor manufacturing process, highly developed processing techniques will not be required of a worker,        because a plurality of sensors can be concurrently formed in a wafer, this process is inexpensive, and suitable for mass production,        since the sensor can be miniaturized, time necessary up to thermal stabilization of the sensor can be shortened,        because the body of a thermal conductivity detector in whole can be miniaturized, constraints imposed on an installation location can be reduced, and        it is possible to concurrently manufacture a variety of thermal conductivity detectors provided with a flow-path, and a filament, for use under various conditions differing from each other.        
A thermal conductivity detector is described in the following Patent Documents 1 and 2.